PDB is an ancient disease, claimed to be identified in a Neanderthal skull;

• Is a common disease (5% prevalence in UK) but most are asymptomatic.
• Primarily a disorder of increased osteoclastic resorption with secondary stimulation of osteoblasts.
  • Pathophysiology: In Paget's disease, the osteoclasts look 'bizzare' on histology AND they have increased sensitivity to vitamin D.
  • Pathogenesis incompletely understood.
  • #autosomalDominant with incomplete penetrance.
• Commoner in older males (onset usually after 55)
  • Thickening of skull bones. -> can lead to hearing loss, increase in hat size
    • Cotton wool appearance on skull X ray.
    • Increased uptake at lesions is seen on bone scintigraphy.
    • Calcium and phophate are normal
    • Bone: Fractures, Bone sarcoma, Deafness (cranial nerve entrapment)

Spondyloarthritis (SpA)

• radiographic changes in the sacroiliac joints in patients with axial SpA
• and asymmetric (oligo)arthritis, heel enthesitis and dactylitis in patients with peripheral SpA
• Rheumatoid factors negative. (<- Seronegative arthropathies)

Inflammatory osteoproliferative lesions in the spine are called syndesmophytes (marginal and non-marginal), and degenerative osteoproliferative lesions are called osteophytes. Syndesmophytes are more vertically oriented than osteophytes.

[!TIP] Mnemonic: RePsAnkIb

• Reactive
• Psoriatic
• Anky spondylos
• IBD associated

Ankylosing = stiffness or fixation of a joint by disease

- Advanced cases: fusion of the sacroiliac joints, and bamboo spine. (Gadolinum MRI is more sensitive than Xray at detecting early changes).

Diagnosis

"Plain x-ray of the sacroiliac joints is the most useful investigation in establishing the diagnosis."

• sacroiliitis is usually the first manifestation 5 and is symmetrical and bilateral
  • the sacroiliac joints first widen before they narrow
  • subchondral erosions, sclerosis, and proliferation on the iliac side of the SI joints
  • at end-stage, the SI joint may be seen as a thin line or not visible

The sacroiliac joint is actually a synovial joint with hyaline on the sacral surface and fibrous cartilage on the iliac surface Source.

• Prevention of syndesmophyte formation -> early diagnosis and initiation of exercise regimen.
• TNF-α blockers - etanercept and adalimumab provide HUGE symptomatic relief. They reduced morbidity

Bone changes seen in various diseases

• Acute, asymmetrical, lower limb oligoarthritis days to weeks after infection such as dysentery or some sexually acquired infections.
  • keratoderma blennorhagica - raised red plaques and pustules. Can occur on soles
  • 
• Majority of patients recover in 6 months but 30% develop chronic symptoms.
• Management is symptomatic. (NSAIDS, corticosteroids, DMARDS like sulfasalazine and infliximab if NSAIDS don't work or a contraindicated)

Langerhans cell histiocytosis

• A multisystem disease
• Birbeck granules present on EM: They have a "tennis racket" shape.
• More common in children (1-3 years of age) but can present in any age even in adults.
• Is one cause of interstitial lung disease.
• Adults presents with pulmonary involvement.
• The disease involves the formation of multiple granulomas throughout the body.
• Adults commonly have lytic lesions in the jaw and skull.
• Pulmonary involvement: also commoner in adults. non productive cough, spontaneous pneumothorax, chest pain etc.
• Upper and middle zone involvement is characteristic.
  Other features:
• bone pain, typically in the skull or proximal femur
• cutaneous nodules
• recurrent otitis media/mastoiditis